ABSTRACT Reproductive biology, size at sexual maturity, and growth
of the deep-water shrimp Solenocera melantho were studied in the East
China Sea. The spawning season continues from July to November, with
peaks during late August to November. Size at sexual maturity
([CL.sub.50]), determined from the proportions of ovigerous females and
of females with maturing ovaries, was estimated at 28.7 mm. The sex
ratio was close to 1:1, with males slightly more abundant than females.
Parameters of growth were estimated by the modified von Bertalanffy
growth function, incorporating seasonal variation in growth. Females
grew faster and reached a larger size at age than males (K = 1.14/y and
[L.sub.[infinity]] = 46.75 mm in carapace length for females, and K =
1.26/y and [L.sub.[infinity]] = 33.6 mm in carapace length for males).
Maximum life span was estimated at 2.43 y for females and 2.20 y for
males. The S. melantho population in the East China Sea has different
values of asymptotic length and growth coefficients in comparison with
other local populations of this species.

Shrimp of the genus Solenocera mostly inhabit offshore waters
ranging from the midcontinental shelf to the ocean floor with depths of
60-1,000+ m (Holthuis 1980, Perez & Kenslery 1997). Currently, 14
species of Solenocera have been reported in the world, and 9 species
occur in the China Sea (Holthuis 1980, Liu & Zhong 1986, Perez &
Kenslery 1997, Song et al. 2006). As a result of the lack of
availability of specimens, little attention has been given to the study
of its fishery, its biology, and its ecology (Chalayondeja & Tanoue
1971, Sunkumaran 1978, Demestre & Abello 1993, Ohtomi & Irieda
1997, Ohtomi et al. 1998, Dineshbabu & Manissery 2008,
Villalobos-Rojas & Wehrtmann 2011).

The deep-water mud shrimp, Solenocera melantho De Man, 1907,
belonging to the Solenoceridae family, is widely distributed from the
Indo-West Pacific region to the Pacific coast of southern Japan (Ohtomi
& Irieda 1997, Perez & Kenslery 1997, Ohtomi et al. 1998, Xue
& Song 2004, Oh et al. 2005). Off the coast of the East China Sea
and the Yellow Sea, the deep-water mud shrimp is one of the most
important decapod crustacean species in terms of total number of
individuals and biomass, and prefers soft (mud and sandy-mud) bottoms at
a depth of 60-100 m (Li et al. 2009, Xue & Song 2004). In China, the
deep-water mud shrimp has been exploited commercially since the mid
1980s, and has developed into one of the main target species for beam
trawls and bottom trawls (Xue & Song 2004).

Information about the reproductive biology of a species is one of
the most important aspects in evaluating the harvesting strategies of
exploited populations. Studies of the reproductive biology of S.
melantho have been reported in Kagoshima Bay, Japan, and Geomun Island,
Korea, and suggest that the reproductive characteristics of this species
have geographical differences (Ohtomi & Irieda 1997, Ohtomi et al.
1998, Oh et al. 2005). In the East China Sea, brief studies on
distribution and biological characteristics of S. melantho have been
conducted by Xue and Song (2004); however, detailed studies on the
reproductive biology of the species are evidently lacking. This article
examines the reproductive biology through determination of the spawning
season, the sex ratio, size at sexual maturity, and growth.

MATERIALS AND METHODS

Specimens were collected from the East China Sea by a 35-m beam
trawl with 25.0-mm mesh cod end (Fig. 1). The sampling was conducted
once or twice a month from June 2009 to May 2010.

Samples were fixed for 24 h in 10% neutral formalin and then
transferred to 70% ethanol for storage. Sex was determined by the
presence of the petasma for males or the thelycum for females (Ohtomi et
al. 1998). Carapace length (CL; from the posterior margin of the orbit
to the middorsal posterior edge of the carapace) and the total length
(TL; from the posterior margin of the orbit to the median margin of the
telson) were measured using vernier calipers (TESA-CAL IP67) to the
nearest 0.1 mm. In a representative subsample, the CL of each specimen
was also measured. Shrimp were weighed with an electric balance
(AWH-SIH) to the neared 0.01 g.

The CL-weight relationships were based on the regression BW =
a[CL.sub.b], where BW is the body weight in grams, CL is the carapace
length in millimeters, and a and b are the constants for each sex
separately. Comparison of the slopes (b) of the length-weight regression
by sex was made by ANCOVA.

For each female, the whole gonads were removed and the ovary stages
were identified according to illustrations of size and color of the
ovary by Ohtomi et al. (1998). Three main stages of development were
observed: (1) undeveloped (ovary transparent, oocyte oogonium), (2)
developing (ovary cream, oocyte early, middle, or late nucleolus) and
(3) early ripe or ripe (ovary yellow or dark yellow, yolk granule or
premature). After blotting to remove excess water, the wet weight of
ripe ovaries was determined by weighing to the nearest 0.001 g using an
electronic balance (OHAUS). The gonadosomatic index (GSI) was calculated
as follows:

GSI = 100 X GW/BW (1)

where GW is wet gonadal weight in grams and BW is wet body weight
in grams.

[FIGURE 1 OMITTED]

Size at sexual maturity was determined by the proportion of females
with early ripe or ripe ovaries. The proportion of mature females by
size was fitted to a logistic equation:

P = 1/(1 + exp(a + bCL)) (2)

where P is the predicted mature proportion, and a and b are the
estimated coefficients of the logistic equation. Parameters were
estimated by correlation analysis of P and CL after linearization. Size
at sexual maturity ([CL.sub.50]), corresponding to a proportion of 0.5
sexually mature individuals, was estimated as the negative of the ratio
of the coefficients ([CL.sub.50] = -a/b) by substituting P = 0.5 into
the equation.

Length-frequency distributions by sex were constructed using 2-mm
intervals of CL. Growth was described using the modified von Bertalanffy
growth function (VBGF) (Pauly & Gaschu 1979, Pauly & David
1981).

where [L.sub.[infinity]] is the theoretical maximum individual size
that the species would reach if it lived indefinitely, K is the
intrinsic growth rate, [t.sub.0] is the age at length 0, C is the
amplitude of seasonal growth oscillation, [t.sub.s] is the age at the
beginning of growth oscillation, and WP (= [t.sub.s] + 0.5) is the time
of year when growth is slowest.

WP ranges between 0 and 1. Values close to 0 or 1 indicate a
deceleration down in growth during the winter months; values close to
0.5 indicate that the deceleration in growth takes place during the
summer. Calso ranges between 0 and 1. For values of C close to 0, the
equation reduces exactly to the von Bertalanffy equation without
seasonality; for values close to 1, the amplitude of the seasonality
factor is maximal.

[t.sub.max] (the corresponding age of [L.sub.max] or maximum
longevity) can be the obtained from the VBGF, and calculated for females
and males:

[t.sub.max] = (2.996/k) + [t.sub.0]

Because the parameters [L.sub.[infinity]] and k are correlated
inversely, the growth performance index ([phi]) was calculated to enable
comparison of growth rates between male and female S. melantho (Pauly
& Munro 1984):

[phi] = 2log[L.sub.[infinity]] + log k

Taking into account the inverse correlation between the two
parameters of [L.sub.[infinity]] and k, the growth performance index is
more robust than either [L.sub.[infinity]] or k individually. Thus, it
fulfills the requirement for a simple single parameter for comparison of
growth.

RESULTS

Sex Ratio

All specimens of S. melantho were sexed, and proportions of females
and males in the samples taken monthly are shown in Figure 2. Of the
1,801 specimens, 49.5% were identified as females and 50.5% as males. A
chi-square showed that the number between females and males was not
different throughout the sampling period ([chi square] = 13.85, P >
0.2), although the sex ratio varied month to month.

The sex ratio of S. melantho varied with CL classes; females attain
a greater size than males. The percentage of females was highest for
more than 25 mm in CL whereas in males it was highest between 15 mm in
CL and 25 mm in CL. A chi-square test showed the number of females and
males was no different when CL was smaller than 15 mm; however, it was
significantly different when CL was larger than 15 mm (Table 1).

Gonad Maturation and GSI

Ovary condition was divided into 3 stages, and females with
immature ovaries occurred throughout the year--mainly from December to
the next June (Fig. 3). Ovarian development began in July, and females
with maturing or ripe ovaries were noted from August to November (Fig.
3).

[FIGURE 2 OMITTED]

All 888 females with different ovarian states were examined to
estimate GSI. The ovary became larger and GSI increased with ovarian
maturity. GSI began to increase in July, and reached its maximum in
October, then began to decrease in December and remained low until the
next June (Figs. 3 and 4). Mature females with a GSI greater than 4
appeared from August to November, which indicates that the spawning
season of S. melantho continues from July to December and the peak is
during August to November. ANOVA showed that there was a significant
difference in mean GSI between months (F = 28.53, P < 0.001).
Turkey-HSD multiple comparisons revealed that the mean GSI of the
breeding season (August to November) was significantly different with
that of other months (Table 1).

Size at Onset of Sexual Maturity)

The 931 female S. melantho used in the analysis ranged from a CL of
13.9-41.1 mm. The minimum CL of mature females was found to be 22.1 mm.
The relationship between CL and the proportion of sexually mature
females by 2-mm-CL size class was calculated by fitting a logistic
function to the size-specific maturity data:

P = 1/(1 + exp(7.84 - 0.24CL)) ([r.sup.2] = 0.89, P < 0.001)

From this, the estimated size for 50% sexual maturity for females
was 28.7 mm in CL (Fig. 5).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

CL-BW and CL-TL Relationships

Using the pooled data sets, a single regression equation, covering
the entire sampling period, was produced for each sex separately. All 3
regressions (females, males, and combined sexes) of BW on CL showed that
the BW was a significantly positive allometric function of CL (Table 2).
Females exhibited higher weight values than males during all study
periods. ANCOVA showed a significant difference in the regression slopes
between sexes (F = 95.61, P < 0.0001).

Between June 2009 and May 2010, 2,009 specimens (1,029 females and
980 males) were collected. The CL of S. melantho ranged from 9.1-41.0 mm
for females and 10.5-31.2 mm for males. The monthly length-frequency
distributions are illustrated separately for each sex in Figure 6.
Statistically significant differences (ANOVA) between the mean CL of
both sexes per month were found (females: F = 103.3, P < 0.001;
males: F = 47.1, P < 0.001).

[FIGURE 5 OMITTED]

In females, the sample in June 2009 was essentially unimodal, with
a peak at a CL of about 20-24 mm. The mode lasted to December with
increasing CL, then young recruits first appeared in January 2010 with a
modal mean CL of 14-18 mm. Males displayed a similar modal progression.

The VBGF parameters, estimated by ELEFAN for each sex, are
summarized in Table 3. The analysis of modal progression for each sex
separately showed that females had lower K values but reached larger
sizes at age than males. This was indicated by the growth performance
indices ([phi]'): 3.28 for females and 3.115 for males. The growth
curve showed a seasonal oscillation in growth (C) of 58% for females and
45% for males. The maximum life span ([t.sub.max]) of S. melantho was
estimated to be 2.43 y for females and 2.20 y for males.

[FIGURE 6 OMITTED]

DISCUSSION

Few biological studies on deep-water shrimp such as Solenocera are
reported in comparison with inshore shrimps. This is obviously
attributed to the difficulty in collecting samples covering a large size
range in deep waters for a long period of time. In the current study, a
large number of specimens off the coast of the East China Sea over a
period of 1 y were collected, and the biological study of S. melantho,
including the sex ratio, spawning season, size at sexual maturity,
CL-weight relationship, and growth, was conducted.

In the current study, the sex ratio of S. melantho was not
substantially different from 1:1, with a little larger value in the main
spawning season (August to November) than that in other months. This is
consistent with previous studies of penaeid shrimp (Ohtomi & Irieda
1997, Cha et al. 2001, Cha et al. 2004a, Cha et al. 2004b, Oh et al.
2005, Dineshbabu & Manissery 2008), and is considered to be a
benefit for the species' life history strategies by raising
reproduction. Although the sex ratio for the total population of S.
melantho was close to 1, it showed varied markedly with CL class, which
has been reported in Solenoceridae (Baelde 1992, Ohtomi & Irieda
1997, Oh et al. 2005, Dineshbabu & Manissery 2008). Possible factors
affecting the sex ratio included longevity, differential migration,
differential mortality, differential growth rate between females and
males, and sex reversal pattern (Wenner 1972, Baelde 1992). However, sex
change was not observed in the current study.

The study of the breeding season in penaeid shrimps can facilitate
our understanding of the adaptive strategies and reproductive potential
of a species related to its environment (Dall et al. 1990, Gillett
2008). And different reproductive strategies, including a continuous
breeding season throughout the year for tropical species and a seasonal
breeding period for subtropical species, have been reported in
solenocerid shrimp (Gueguen 1998, Ohtomi et al. 1998, Ohtomi &
Matsuoka 1998, Oh et al. 2005, Dineshbabu & Manissery 2008). The
breeding season of S. melantho was reported by several researchers in
different areas. Ohtomi et al. (1998) found that the species inhabiting
Kagoshima Bay, Japan, had a breeding season from June to December, with
a peak during October to November. Oh et al. (2005) estimated the
breeding season of the same species in Geomun Island, Korea, from August
to early November, with a peak in October to early November. The current
study clearly showed that S. melantho had a single breeding season from
August to November, as reflected by the ovaries of mature females and
monthly GSI values. All of the previously mentioned studies on S.
melantho indicated that, regardless of where it was found, it exhibited
1 spawning peak per year, and the peak spawning season tended to be
longer in lower latitudes (Table 4). The bottom temperature in this
study fluctuated from 12.8-21.9[degrees]C, and the mean value is a
little higher than that in Kagoshima Bay and Geomun Island. This
difference may lead to the variance in length of spawning season, as
discussed in previous studies (Dall et al. 1990, Hossanin & Ohtomi
2008). However, more data are needed to understand more completely the
influence of factors controlling the variance in spawning of S.
melantho.

Size at sexual maturity is very important in fisheries regulations,
determining the size limits with the aim to protect immature individuals
from exploitation and to ensure an adequate number of reproductively
active animals to support the total reproductive output, or egg
production, of the populations (Lizarraga-Cubedo, 2008). This study
determined that the size at sexual maturity in female S. melantho was a
CL of 28.7 mm (estimated as 101.4 mm in TL). The value was much larger
than that reported in previous studies (Ohtomi et al. 1998, Oh et al.
2005) (Table 4). In penaeid shrimp, 2 methods--including histological
characteristics and ovary maturation--are used to estimate the size at
sexual maturity (Ohtomi et al. 1998, Cha et al. 2001, Oh et al. 2005,
Dineshbabu & Manissery 2008). Ohtomi et al. (1998) considered the CL
of the smallest mature female with stage-III ovaries among the specimens
as the size at sexual maturity. The difference in criteria may result in
the difference in determining the size at sexual maturity. Geographical
variations may be another influence determining the size at sexual
maturity, which is well documented for other decapods, including Jasus
edwardsii (Annala et al. 2010, Gardner et al. 2006), Marsupenaeus
japonicas (Ohtomi et al. 2003), and Penaeus indicus (Jayawardane et al.
2002). The variations could be the result of differences in population
abundance and structure because of variations in the characteristics of
catchability of fishing gears used on different grounds, or differences
in growth rates between areas.

In the current study, negative allometry was observed in the CL-BW
relationship in both males and females, with a faster growth rate in
females than males. This result agrees with many previous studies (Oh
& Hartnoll 2004, Josileen 2011, Ozcan & Katagan 2011). Penaeid
shrimp typically show an allometric coefficient (b) close to 3. The
value in this study was a little less than that in S. melantho (Ohtomi
& Irieda 1997), and larger than that in Solenocera membranacea
(Ozcan & Katagan 2011). However, our result was within the limits of
2.5-3.5 reported by Froese (2006).

This study suggests that the first recruitment of S. melantho in
the East China Sea occurs once a year in winter (January), and similar
patterns were reported in earlier studies of the same species in
Kagoshima Bay populations (Ohtomi & Irieda 1997). The von
Bertalanffy growth models fit to the data of S. melantho, as indicated
from the high score function ([R.sub.n]). The estimated values of K
(1.14 for females and 1.26 for males) and [L.sub.[infinity]] are
different from that of a population in Kagoshima Bay (Ohtomi &
Irieda 1997) and Geomun Island (Oh et al. 2005) (Table 3). However, the
maximum age is similar between different populations of the same
species. The K values decreased within the range (0.39-1.6) reported by
Pauly and Munro (1984) for a number of penaeid shrimp. The variations in
growth parameter were suggested to be attributed to sea water
temperature and fishing pressure (Taylor 1958, Oh et al. 1999). The
growth of performance index ([phi]') is preferred for growth
comparison between species and sexes of a species rather than
comparisons of K and [L.sub.[infinity]] because of the inherently
negative correlation between these 2 parameters (Pauly & Munro
1984). Our [phi]' values, which changed a little in different
areas, were closely consistent with those in different populations
(Table 3). The comparison of [phi]' between sexes indicates that
the growth rate is larger and reached a larger size for females than for
males, with relatively low growth in both sexes during the spawning
season. The coincidence of slow growth and maturation with spawning
periods in females indicates that metabolic costs are associated with
reproductive activities, when shrimp cease molting during the spawning
period. Similar patterns of growth are found in previous studies in
penaeid shrimp (Baelde 1994, Ohtomi & Irieda 1997, Ohtomi &
Matsuoka 1998, Cha et al. 2001, Lopez-Martinez et al. 2005, Oh et al.
2005, Hossain & Ohtomi 2008); however, this pattern did not appear
to be generalized in decapod crustaceans (Hartnoll 1982).

ACKNOWLEDGMENTS

We thank Professor Li Ping Yan for helpful advice and suggestions
in field sampling. This work was supported by special research funds for
the national nonprofit institutes (East China Sea Fisheries Research
Institute; no. 2007M17), the Chinese Ministry of Agriculture Assessment
of Marine Fisheries Resources Program, and the Ministry of Science and
Technology Public Project (2008-2010).